1. Field of the Invention
The invention relates to preform elements of the kind comprising a facing table of polycrystalline diamond bonded to a substrate of less hard material, such as cemented tungsten carbide.
2. Description of Related Art
Preform elements of this kind are often used as cutting elements in rotary drag-type drill bits, and the present invention is particularly applicable to the treatment of such preform cutting elements before they are mounted on the drill bit. However, the invention is not restricted to cutting elements for this particular use, and may relate to preform elements for other purposes, for example as cutters on roller cone and percussive bits. Also, preform elements of the kind referred may also be employed in workpiece-shaping tools, high pressure nozzles, wire-drawing dies, bearings and other parts subject to sliding wear, as well as elements subject to percussive loads as may be the case in tappets, cams, cam followers, and similar devices in which a surface of high wear-resistance is required.
Preform elements of the kind to which the invention relates are generally manufactured by pre-forming the substrate in the appropriate required shape, applying a layer of diamond particles to the surface of the substrate and then subjecting the substrate and diamond layer to very high pressure and temperature in a forming press so that the diamond particles bond together and the diamond layer bonds to the substrate. This manufacturing process is well known and will not be described in detail.
Each preform cutting element may be mounted on a carrier in the form of a generally cylindrical stud or post received in a socket in the body of the drill bit. The carrier is often formed from cemented tungsten carbide, the surface of the substrate being brazed to a surface on the carrier, for example by a process known as "LS bonding". In this process the diamond facing layer is cooled while the surface of the substrate is brazed to the carrier. The reason for this is that the polycrystalline diamond facing table is not thermally stable above about 725.degree. C. and will begin to graphitise. The cooling of the facing table is therefore necessary, since the brazing process would otherwise raise the temperature of the preform element above this critical temperature.
Such preform cutting elements are subject to high temperatures and heavy loads when the drill bit is in use down a borehole. It is found that as a result of such conditions cracking of the substrate can occur. Also, spalling and delamination of the polycrystalline diamond facing table can occur, that is to say the separation and loss of the diamond over the cutting surface of the table. This may also occur in preform elements used for other purposes and particularly where the elements are subjected to repetitive percussive loads, as in tappets and cam mechanisms. It is believed that the tendency for cracking, spalling and delamination to occur depends in part on the residual stresses which are left in the preform element at the end of the process of forming the element and fitting it to the bit body. It is therefore common practice to heat-treat the preform elements after formation in the press and before mounting on the bit body in order to relieve some of the residual stresses in the element and thereby reduce the tendency of the elements to crack or delaminate in use.
One common method of heat treatment, designed to achieve thermal stress relief, is to maintain the preform elements at temperatures of up to 500.degree. C. for a considerable extended period, for example up to 48 hours. However, while this is believed to have some stress relieving effect, subsequent cracking and delamination of the preform elements may still subsequently occur.
The present invention provides a new form of heat treatment for preform elements which may not only achieve more effective thermal stress management, but which also reduces the time cycle for manufacturing each element and thus enables output to be increased.